NBS Measurement Services:

Radioactivity Calibrations

with the "47r" Gamma Ionization

Chamber and Other Radioactivity

Calibration Capabilities

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National

Bureau of Standards. The Center provides the Nation with standards and measurement services for ionizing

radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support

to the National Measurement Support Systemfor ionizing radiation; conducts research in radiation related

fields to develop improved radiation measurement methodology; and generates, compiles, and critically

evaluates data to meet major national needs. The Center consists of five Divisions and one Group.

Atomic and Plasma Radiation Division

Carries out basic theoretical and experimental research into the • Atomic Spectroscopy

spectroscopic and radiative properties of atoms and highly ionized • Atomic Radiation Data

species; develops well-defined atomic radiation sources as radiometric • Plasma Radiation

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

Provides the central national basis for the measurement of far ultra- • Far UV Physics

violet, soft x-ray, and electron radiation; develops and disseminates • Electron Physics

radiation standards, measurement services, and data for for these radia- • Photon Physics

tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applications; determines fundamental mechanisms of electron and photon interactions with matter; and develops advanced electron- and photon-based

measurement techiques. The Division consists of the following Groups:

Radiometric Physics Division

Provides national measurement standards and support services for ultra- • Spectral Radiometry

violet, visible, and infrared radiation; provides standards dissemination • Spectrophotometry

and measurement quality assurance services; conducts research in optical • Radiometric Measurement Services

radiation, pyrometry, photometry, and quantun radiometry; and develops

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Develops, operates, and improves major NBS radiation facilities

including the electron Linac and race track microtron; develops,

designs, and builds electronic and mechanical instrunentation for

radiation programs and facilities; provides national leadership

in the standardization of nuclear instriitientation; and develops

new competence in radiation sources and instrunentation. The

Division consists of the following Groups:

• Accelerator Research

• Linac Operations

• Electronic Instrumentation

• Mechanical Instrunentation

Ionizing Radiation Division

Provides primary national standards, measurement services, and basic

data for applications of ionizing radiation; develops new methods of

chemical and physical dosimetry; conducts theoretical and experimental

research on the fundamental physical and chemical interactions of

ionizing radiation with matter; provides essential standards and measurement support services to the National Measurement Support

System for Ionizing Radiation; and develops and operates radiation

sources needed to provide primary radiation standards, fields, and

well-characterized beams of radiation for research on radiation

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research

Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

understanding of physical processes that underlie measurement science.
NBS MEASUREMENT SERVICES:

RADIOACTIVITY CALIBRATIONS

WITH THE "471" GAMMA

IONIZATION CHAMBER

AND OTHER RADIOACTIVITY

CALIBRATION CAPABILITIES

Jacqueline M. Calhoun

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Clarence J. Brown, Acting Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1987
Library of Congress Catalog Card Number: 87-619870

National Bureau of Standards Special Publication 250-10

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-10, 41 pages (Oct. 1987)

CODEN: XNBSAV

Commercial products— materials and instruments— are identified in tliis document for the sole pur- pose of adequately describing experimental or test procedures. In no event does such identification imply recommendation or endorsement by the National Bureau of Standards of a particular product; nor does it imply that a named material or instrument is necessarily the best available for the purpose

it serves.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1987

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

The calibration and related measurement services of the National Bureau

of Standards are intended to assist the makers and users of precision

measuring instruments in achieving the highest possible levels of

accuracy, quality, and productivity. NBS offers over 300 different

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services

are offered to the public and private organizations alike. They are described in NBS Special Publication (SP) 250, NBS Calibration Services

Users Guide .

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed description of the important features of specific NBS

calibration services. These documents provide a description of the:

(1) specifications for the service; (2) design philosophy and theory;

(3) NBS measurement system; (4) NBS operational procedures; (5)

assessment of measurement uncertainty including random and systematic

errors and an error budget; and (6) internal quality control procedures

used by NBS. These documents will present more detail than can be

given in an NBS calibration report, or than is generally allowed in

articles in scientific journals. In the past NBS has published such

information in a variety of ways. This series will help make this type

of information more readily available to the user.

This document (SP 250-10), NBS Measurement Services: Radioactivity

Calibrations with the "4it" Gamma Ionization Chamber and Other

Radioactivity Calibration Capabilities, by J. M. Calhoun, is the tenth

to be published in this new series of special publications. It describes the use of the NBS "4tt" gamma ionization chamber to measure

the activity of gamma-ray-emitting radionuclides against national

standards (see test numbers 43010C and 43020D in the SP 250 Users

Guide). It also reviews NBS capabilities for making direct

radioactivity calibrations. Inquiries concerning the technical content

of this document or the specifications for these services should be

directed to the author or one of the technical contacts cited in SP

250.

The Center for Radiation Research (CRR) is in the process of publishing

21 documents in this SP 250 series, covering all of the calibration

services offered by CRR. A complete listing of these documents can be

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

be made more useful. Suggestions are also welcome concerning the need

for new calibration services, special tests, and measurement assurance

programs

George A. Uriano Chris E. Kuyatt

Director Director

Measurement Services Center for Radiation Research

i i i
ABSTRACT

This paper describes the use of the NBS "4u"y ionization chamber — an instrument which provides an indirect method of comparing the activity (decays

per second) of gamma-ray-emitting radionuclides with national standards for a routine calibration service by the National Bureau of Standards Radioactivity

Group. A description of the chamber's construction and characteristics, the

operational procedure, and the associated equipment is included.

A description of NBS capabilities for direct radioactivity calibrations

is also presented. Many of these capabilities are used to establish

calibration factors for the "4tt"y ionization chamber.

Key Words: activity; calibration; gamma ray; ionization chamber;

radionuclides; radium reference source; standard
TABLE OF CONTENTS

Page

Abstract i v List of Tables vi

List of Figures vi i

1. Introduction

1.1 General Information: 1 1.2 Description of Service 1 1.3 Specifications for Samples Submitted for Calibration in the

NBS "4tt"y Ionization Chamber 2

2. Design Philosophy and Theory

2.1 Physical Principles and Restrictions 5

2.2 Direct Calibrations 6

2.3 Comments on Direct Activity Measurements 9

2.4 Nuclear Radiations and Direct Activity Measurements 9

2.5 Direct Methods Using Calculated Efficiencies 10

2.6 Direct Methods Using Cascade Radiations 11

2.7 Radioactivity Group Instrumentation at the U.S. National

Bureau of Standards 13

2.8 Present Basic Radionuclide Calibrations at NBS 13

3. Principles of Operation

3.1 Construction of the NBS "4Tr"Y Ionization Chamber 13

3.2 Characteristics and Functions 21

3.3 Current Measurements 21

3.4 Source Container and Contents 22

4. Operational Procedures

4.1 Description of the Actual Measurement Process 22

4.2 Receiving Instructions for Radioactive Solution Samples 23

4.3 Step-by-Step Operating Instructions for the NBS "4tt"y Pressure

Ionization Chamber 23

4.4 Data Analysis 26

4.5 Format of Calibration Report 27

5. Assessment of Uncertainty

5.1 Precautions and Uncertainties in Service Calibrations 27

5.2 Significance of Uncertainty Statements 30

5.3 Examples of Uncertainty Components 31

6. References 33

V
LIST OF TABLES

Page

Table 1 Specifications for Calibration of Gamma-Ray Emitting Solutions

Which are Submitted for Calibration (Radionuclides having half

lives greater than 15 days) 3

Table 2 Specifications for Calibration of Gamma-Ray Solution Sources

(Radionuclides having half lives less than 15 days) 4

Table 3 Instruments for NBS Basic Calibrations Not Using Cascade

Radiations 14

Table 4 Instruments for Basic NBS Calibrations Using Cascade Radiations 16

Table 5 Radionuclide Measurement Capabilities in the Radioactivity

Group at the U.S. National Bureau of Standards 17

Table 6 Sensitivity Range Settings 25

vi
LIST OF FIGURES

Page

Fig. 1. Sectional View of a One-Inch Diameter reentrant-well 7 Fig. 2a. Relative Response of Chamber vs. Gamma-Ray Energy 0-500 keV.... 8

Fig. 2b. Relative Response of Chamber vs. Gamma-Ray Energy 500-3000 keV. 8

Fig. 3. Schematic Diagram of Electronic System for the "4Tr"Y

Ionization Chamber 21

vi i

1. Introduction

1.1. General Information

The N6S "4Tr"Y ionization chamber ( IC) is a wel 1 -type pressurized

instrument which is used for measuring the activity (decays per second) of

gamma- ray-emitting radionuclides. Scheduled calibration services for

submitted samples of gamma-ray-emitting radionuclides are offered as a complement to the Standard Reference Material (SRM) program as described in

NBS Special Publication 260 [1], These calibration services are advertised in

the NBS Special Publication 250 (SP250) [2] as the "old" 8.2C (for radionuclides with half lives greater than 15 days)and 8. 2D (for radionuclides with

half lives less than 15 days) which are now the "new" 43010C and 43020C,

respectively. The range of activity for the gamma-ray-emitting radionuclides

is 0,4 - 60 MBq, and the nominal uncertainty range is 0.7 - 3.4 percent

(corresponding to 3 standard deviations). Special tests for beta-particleemitting solution sources are also available. These special tests are advertised in the SP250 as the old 8.2Q, R, and presently 43060S and 43070S,

The specifics of these special tests are not discussed in this paper, but

general information is given about the direct calibration of the beta-particle

emitting radionuclides in section 2, Design Philosophy and Theory.

The "4tt"y chamber uses calibration factors for each radionuclide

previously established by direct methods of activity measurements (1,2). A

description of the instrumentation used to make direct measurements is given

in the section on Design Philosophy and Theory. Many IC calibrations have

been checked with those of other national standardizing laboratories through

the Bureau International des Poids et Mesures (BIPM).

The gammaionizationray-emitting radionuclides

chamber calibrations are: which may be submitted to NBS for "4Tr"

Na-22 Fe-59 Mo-99 1-123 Ba-140 Au-195

Na-24 Co-60 Tc-99m 1-131 Ce-141 Hg-197

Sc-46 Ga-67 Cd-109 Ba-133 Eu-152 Au-198

Cr-51 Se-75 Ag-llOm Cs-134 Eu-154 Tl-201

Mn-54 Sr-85 In-Ill Cs-137 Eu-155 Hg-203

Co-57 Y-88 Sn-113 Ce-139 Yb-169 Pb-203

1.2 Description of Service

1.2.1 Purpose and Results of Service . The calibration service for the

ganma-ray-emi tti ng radionuclides is available for the technical user who must

make measurements consistent with national standards or who requires higheraccuracy calibrations than are available with commercial standards. When the

calibration is completed, the technical user is provided with a report

stating:

1
a) the principal radionuclide

b) reference time and date

c) method of calibration

d) certified value of activity

e) decay-scheme assumptions

f) assessment of radionucl idic purity

g) overall uncertainty

When the calibration service is completed, the technical users are notified,

and the calibrated sample returned, if desired by customer. Short-lived

radionuclide samples are usually measured by the customer only before

transmittal to NBS, for the activity would be too low for measurement after

the NBS calibration.

1.3 Specifications for Samples Submitted for Calibration in the NBS "4tt"y

Ionization Chamber

1.3.1 Submitted Solution Specifications . Samples submitted for calibration must be chemically and physically stable. Tables 1 and 2 list the

chemical forms which are suggested for these solution sources. Tables 1 and 2

also list the activity ranges and their associated nominal uncertainties.

The solution sample should consist of a specific radionuclide in

5.0 ± 0.2 milliliters of liquid flame-sealed in a NBS-type borosi 1 i cate-gl ass ampoule. The ampoule will be provided by the Radioactivity Group.

2
Hi

z

o

h-3

-Io

(0

o

il o >•

UJ < u) K £

uE

o

u

Ee£O

o

o

Me

o>

O)3

0)

e < o

z t 5 <

H§OnI

£ uj S

o T

(0 ^

<

o

>>xs _ c ^ cr

.2 0>ffi < oc

®

C jQ « ^

- S

<D 1O

O £

^ 9 JS o c rS c « 3 a O

N

o

a.(0

o o oo_ o ^ z

O O X O O O O Z O O X Z U OOOOOOOOX77X

X X 2 X X X X X X X ^ X X X X X X X X X X 2 ^ SSu)SSSSSZS(oZS S S S Z S S S S ^ o^T0000000000000

I I I I I I I I I I I I I o o o o

SOOOOOOQQQ CO CO <0 CO ^ ^ (D CO CO

I I I I I I I I I I I CNCMCMCMM^^CMCMW CM d d

^0^ ^ ^ ^tf^ ^

tt>eooc\ioq^eo^Oh«h^o>p

i-^dT-'T^dr^dcMCMdr-'deo

LOOLOOocoooinmco «

T^T^T^T-'CMT^dT^Oioi

ZMOSUU.U(A(0>»o auk.coa>oa>w OEMm

O) o> „ < < £

I

° E

en ^ CO O ^

I

I o> I o < C

(S

flO E

I m U O O O UJ UJ UJ >- < X

3

oO

U

M

C

o

CB

• ^ Ei» a—

o n «. o I'« 0) £ CD 0 Z

•o *. C (0 " M

1|

> c

1 ^

g g e o S CO c (0 o

® iJ e

• c u o ~ o = «

•* •= £ W

>>£ Z

ceo •5 "2 £

r 5 ^

• o ?!

U £

c •

o

** 2z

2 £

5 «

• 3

i|

is

2 s

« 3

•S t

C eg CB o W •

</) » ^' O

><

o

COc

CB

w

(0E«

>>

CBw

CBE

E

CB

i2 o

3 *: 0

e

•* <fl C CB o c r «

S -c 9 S

CO « 0 E

uc " CB

CB ^ C

1 O P

A 4> O _ O O

iS 3 •

o o £

o e 2

I- »- E

M

(0 5|

— 3

c e

o o

<

E

I o

u

PC c

o S

£ O

, £

U CB

CB U

C «

O £

3
CO

UJIo

(0—

w

>» CO 5

>- if) < ^ s s O o> U. = Zg g>

I- > s

CD S <Oo

u.COzg<o

UJ

Q,

CO o3

•o

(0 oEo

<0o1oo

(/) 0)O)

O)3 coo

0>

(0 o

CM

LU

_l

GQ< > 0)ffi < cc o

CO ^ (0 o o -c .OSN

T3c

(D

CO o o o X X

T- CM « * X X o o o ^ ^ _ CO Z o c

75o

PP (0 « (0 •o c n 0) >» o o (0

0) oO Q. z oX

O

X I I

CO o _ z o X X

CO o q 2 O O 1-O "t- 1- o o «o o o CO CO z 0 0=-

C4 (/>(/) <0

OJ CM_J

O (0 CO „ c Z Z j:; ^ „ CO o

CM ^

3^z X ^

CO OoZ m o o o o ^ ^ (O (O I I I I ^ ^ CM CM d d o o o o o o

<0 (O (O ^ (O (O I I I I I I CM CM CM ^ CM CM

OO

(O (O I I CM CM

>P vO

op ^ (C> in d

vO vO ^ nO vO

CO LO CO ^ ^ CO

>5

T- 1- T- CO CVJ T- ^ 1- E

O)

O)

(Q (0

O ^ Z O S E

Tt 1^ O) C7> CM (o a> a>

CB _lOI _ (0 0> 3 _ ^ £ OQX< HO. T— CO T- O 00 T— CO

T- CM CO ^ cn o) oo f- y- r- y- y- y- CMCM c3

•oo£oco« E ^ k- — 0 (0o 8 -c 1 (/) £ OQ « Z

-oC (0 ll

M 'O .2 w

IS2 s

Jo o o

O) (0 § S c o 2 o 3 ^© — c <o o © .2 "E

a> c o _ © ••- •=£</) S *- CO >».E Z c © ©

-o o © CO

CO > c

«5 2 2 0) © o _ o o S3© © © !S P P E P© 2 O 3 C CO n o £ B

CO c (0 o c r «

CO £ © ^ © ©

•o E S «> JSs w o — 3 c c o o

CO ^ E 2 P CO ^ c S o — £ «

.2

r ? «- o o

u.

u 4
1.3.2 Packagijig. Packaging for all sources must be in compliance with

the Department of Transportation (DOT) and Nuclear Regulatory Commission (NRC)

regulations. Copies of these regulations may be obtained from Operations

Division, Office of Hazardous Materials, Department of Transportation,

Washington, DC 20590. Postal regulations prohibit mailing radioactive

materials which require a caution label under DOT regulations.

1.3.3 NBS Shipping Address . All samples submitted for calibration are checked by the NBS Health Physics Group for radiation level and source integrity before transferring to the NBS Radioactivity Group for calibration.

The following shipping address must be used:

National Bureau of Standards

Atten: Name of NBS contact

Health Physics (Radioactivity Group)

Quince Orchard and "Clopper Road

Gaithersburg, Maryland 20899

2. Design Philosophy and Theory

2.1 Physical Principles and Restrictions

The function and status of the "4Tr"Y ioni zation-chamber calibration

service can be better appreciated if the difficulties in maintaining national

standards for the activity of radionuclides is considered. Each of the 80-100

radionuclides for which activity calibrations are sometimes required in the

United States presents a separate challenge for a "direct" calibration,

independent of existing standards. Such calibrations, discussed in a section

to follow, are usually time-consuming and not applicable to routine submitted

samples. Because radionuclides by definition decay, a comparative measurement

against an existing, laboriously directly- calibrated, sample may not be

possible after even a few days.

What is needed is a stable, easy-to-use instrument to serve as a repository once the direct calibration has been performed for a particular

radionuclide. With due precautions, an ionization chamber serves this purpose

if gamma rays of sufficient energy and probability are emitted as part of the

decay process. Gamma-ray interactions in a gas, pressurized to maximize their

probability, produce ion-electron pairs in proportion to the gamma-ray energy.

A voltage between two electrodes causes these charges to be collected; the

resulting current is also proportional to the number of decays per second,

called the "activity" and assigned the name bequerel (Bq). One bequerel of a given radionuclide always produces the same current if all conditions are the

same. Hence a calibration, or "K" value can be assigned for that radionuclide, on a particular ionization chamber.

As a precaution against changes in the chamber (loss of gas; movement of

electrodes) or the sensitive electronics required to measure the small

currents produced by the usual -activity samples, the current for the sample is

measured relative to that from a long-1 i ved-radionucl ide sample which was also

measured when the original "K" value was determined. To account for possible

changes in leakage currents or differences in external radiation, "background"

measurements with no sample are also interleaved with sample and reference

source runs during a measurement series.

5
Samples which are too low in activity produce a current insufficiently

different from background; samples which are too strong may produce so many

ion pairs that recombination can occur before the electric field sweeps them

away. Any change in container, solution nature, or density can cause changes

in gamma-ray absorption. Sample location must always be the same; in order to

achieve this, and to maximize the current, the chamber is made re-entrant so that the sample can be placed at the center (see fig. 1).

The chamber cannot distinguish between currents produced by different

radionuclides. Hence all samples must be examined with calibrated gamma-rayspectrometry systems, and the effect of any impurity radionuclides removed.

The amount (activity) of the impurity present is deduced from the spectrometry

measurements, but a calibration factor may not have been established for some uncommon radionuclides. In this case, a response (current) can be synthesized

from the observed gamma-ray-emission rates at different energies and an ionization chamber "response" curve constructed from the measured response of

radionuclides emitting one or two gamma rays of known probability per decay

(see fig . 2a and 2b)

Obviously, the ionization chamber is not suitable for radionuclides which

emit only alpha particles or electrons, or for which NBS direct calibrations

of sufficient accuracy have not yet been performed. However, the chamber

offers a useful means of propagating national standards for, at present, some

36 radionuclides, including many for which SRM's are not regularly available.

2.2 Direct Calibrations

The following excerpt is from an article [1] that summarizes most of the

direct calibrations which have been performed at NBS, including those used in

establishing the National Bureau of Standards IC calibration values. All

methods and radionuclides are given to indicate other possible NBS services

beyond those listed in SP250. The uncertainties given in the article,

obviously different for each radionuclide, usually contribute the major uncer- tainty in a calibration certificate. A change in the method of specifying the

uncertainties retains their "3a" significance, as given in this article on pages 40-44. Detailed descriptions of the direct activity measurement

techniques are given in Ref. [2].

6
I

I

Fig. 1. Sectional view of a one-inch diameter reentrant-well IG = 11 pressure

ionization chamber. (From Centronic, 20th Century Electronics Ltd.,

King Henry's Drive, New Addington, Croydon CR9 OBG, England.)

7
ENERGY, keV

Fig. 2a. Relative Response of Chamber vs. Gamma-Ray Energy 0-500 keV.

500 1000 1500 2000 2500 3000

ENERGY, keV

Fig. 2b. Relative Response of Chamber vs. Gamma-Ray Energy 500-3000 keV.

8
2.3 Comments on Direct Activity Measurements

All direct measurements depend to some degree on existing nuclear-decayscheme data. For example, calorimetric measurements require a knowledge of the

average energy per decay. The associated uncertainty for this method is

trivial for some radionuclides emitting only alpha particles of a single

energy which has been measured to five significant figures, but can be quite

significant for beta decays for which the spectral distribution and maximum

energy have not been measured well. Many of the methods developed for radionuclides with cascade radiations do not depend sensitively on details of the

decay. In this article it is accepted that a requirement for nuclear data

will not rule out a method being called direct if the sensitivity to those

data, and the certainty of the values, will permit an acceptable overall

uncertainty. Usually at least one nuclear datum, the half life, is required if

the measured value is to have significance at any earlier or later time.

It should also be noted that all of the direct methods assume that no impurity radionuclide is present. This can be tested by repeated measurements

over an appreciable fraction of a half life, but in practice a careful search

for impurities by other means is a prudent first step in any calibration.

Another very important topic not discussed here is the quantitative preparation of suitable and stable sources and solutions.

Techniques discussed here are assigned, somewhat arbitrarily, to one of

two general classes: (1) those where the detector efficiency can be calculated from geometrical and physical information, without the use of

information obtained from cascade radiations; and (2) those where the cascade

radiations form the basis of the method.

2.4 Nuclear Radiations and Direct Activity Measurements

In order to relate the counting rate or current from a detector to the

activity of a sample of a radionuclide being measured without use of a reference standard, several points usually have to be addressed: (1) What is

the radiation to be detected, what is its energy, what is its probability per

decay, and what is the effect of other radiations?; (2) What is the effect of

the source matrix and support?; and (3) What fraction of the emitted

radiations interact in the detector and what is the response?

Some of the methods discussed later can bypass these questions, but to see the difficulty in devising methods to answer the questions with sufficiently small uncertainty, it is useful to consider the properties of different

radiations. Alpha particles are absorbed readily in thick sources and are accompanied by nuclear recoils that can cause confusion, but they have large

and unique energies, backscatter from supports only at near-grazing angles,

and can be restricted by a defining aperture with little contribution from

edge effects. Beta particles are emitted with a spectrum of energies from

zero to some characteristic maximum energy, with energy-dependent absorption

or scattering in sources or supports. Positrons share these characteristics

and produce annihilation photons. Positron decays also have electron-capture

decays competing. Electron capture, which happens even when there is insufficient energy for positron emission, can occur in different atomic shells, and

the subsequent atomic deexcitation produces both characteristic x rays and

Auger electrons.

9
Subsequent to the primary decay, excited levels in the daughter nuclide

then decay by gamma-ray emission or by internal conversion. The latter pro- cess also gives rise to x rays and Auger electrons. Long-lived excited states

can contribute extra counts for the fraction of decays occurring beyond the

resolving times of counting circuits. Radionuclides decaying only by isomeric

transitions, without alpha or beta radiations, must also be calibrated.

The many types of radiations, and the large range of energies and half

lives encountered, suggest that there is no universal method for direct

calibrations, but rather that each radionuclide must be considered as a separate challenge. Also, most standardizing laboratories consider it prudent

to check a calibration with an alternate method wherever possible. The

reduction in the uncertainty of nuclear-decay data in recent years, together

with improvements in technique, make measurements with efficiency-calibrated

germani um-spectrometry systems a common indirect way of making a final check

for many radionuclides.

2.5 Direct Methods Using Calculated Efficiencies

Methods using calculated efficiencies can be divided into those for which

the efficiency is very high ("4t:" methods) and those for which the efficiency

is smaller, usually because the solid angle is appreciably smaller, but where

the latter can be specified with sufficient accuracy ( "def i ned-geometry " methods). Both methods carry a requirement that at least the sum of the

probabilities per decay for each of the radiations to be detected must be

known with an acceptable uncertainty. Both usually require some extrapolation

or correction for events too low in energy to be separated from system noise.

2.5.1 High-Efficiency Counters . Two instruments commonly used for "4tt"

measurements are gas-proportional counters and liquid-scintillation counters.

In the former, attenuation in the source and backing materials is the usual

limitation for solid sources. In the latter, difficulties may arise from

extrapolation complexity, resolution limitations, "quenching" (reduction of

light output with the addition of the radioactive material), and the

sensitivity to radiations other than the ones desired.

For radionuclides emitting beta particles of sufficiently high energy,

4tt proportional counters can yield results with a satisfactory uncertainty.

Suitable precautions in preparing thin conducting support films and thin sources must be observed.

For alpha particles, liquid-scintillation counting avoids the problem

of energy loss in a support, but any energetic electrons may interfere with a measurement because of the much higher light output for a given energy absorption relative to alpha particles. If such interferences are taken into

account, simple liquid-scintillation counting can be quite effective.

2.5.2 Calorimetry . If the average energy per decay for a radionuclide

is knpwn and almost all of this energy can be transformed into heat in a calorimeter, measurements of low uncertainty are possible, even for

radionuclides emitting low-energy beta particles.

10
2.5.3 Internal Gas Counters . Radionuclides in the gaseous form can be

introduced directly into the counting gas of proportional counters. With

precautions for quantitative mixing and handling, and compensation for

boundary effects, low uncertainties can be obtained even for rather low-energy

beta spectra.

2.5.4 Def i ned-Geometry Counters . Counters with a 2-n geometry are widely

used in routine counting, but backscatteri ng from a thick backing often makes

them unsuitable for a direct measurement of beta particles, except under

special circumstances. However, in the case of alpha particles, quantitative

investigations of source and backing scattering have provided sufficient

information so that an activity can be deduced from a 2ir alpha-counting rate

with moderate accuracy. The predominant emission of backscattered alpha

particles almost parallel to the backing suggests, however, that more restricted geometries will give results of lower uncertainty.

Def i ned-geometry counters' with restricting apertures are not usually

considered suitable for electron counting because of the ill -defined

scattering from edges. They are suitable for emission-rate measurements of

low-energy photons, but the large uncertainties in probabilities per decay

usually prohibit conversion into useful activity values.

2.6 Direct Methods Using Cascade Radiations

The methods discussed so far are most often used for radionuclides

without appreciable branching to excited nuclear levels. However, more complex decays can give enough information to yield efficiency values for the

detectors as part of the activity measurement. The essence of most methods is

that not only are two radiations detected, each with some unknown efficiency,

but the simultaneous detection of both (which depends on the product of

efficiencies) is also recorded or inferred. With three measured quantities

expressed as functions of the efficiencies of the two detectors and the

activity of the sample, enough information exists for the three unknowns to be

calculated. As with the previously discussed counting methods, the effects of

backgrounds, dead-times, and pulse pileup must be considered, but with cascade

radiations there may also be timing and resol vi ng-time considerations.

2.6.1 Coincidence Counting . For a simple alpha-gamma or beta-gamma

cascade, with each of two detectors sensitive to one radiation and not the

other, corrections and analysis are straightforward. Many systems using

proportional or liquid-scintillation counters to detect the particles, and

Nal(Tl) or germanium detectors for photons, have been described.

Usually analysis is simplified if the detector for the particles has a high efficiency, but sometimes the circumstances require other choices. The

reduction in gamma-ray rate due to branching from a level, or to internal

conversion, is incorporated into an apparent detection efficiency, but

explicit corrections must be made for other branchings from the level originating the cascade. Extra counts in the particle detector due to the detection

of gamma rays or i nternal -conversion electrons must be taken into account.

The effect of dead times for moderate counting rates was treated approximately

for many years with small error, but recent formulations allow accurate

correction even at high rates.

1,1
2.6.2 Anticoincidence Counting . If the efficiency of one detector is

very high, then it is reasonable to count those pulses from the other detector

which do not have a coincident pulse. As applied, this method has the

technical advantage that the timing is not critical and the effect of

long-lived states, which is difficult to measure by the coincidence method,

can be measured directly.

2.6.3 Anticoincidence and Coincidence Counting with Efficiency

Extrapol atioru One must normally correct for other radiations that can produce counts in the beta detector when a beta particle is not detected. As

the efficiency for detection of the beta particle (or positron or Auger

electrons and x rays) approaches unity, however, these corrections go to zero. Thus one can introduce solids into the source or add absorbing films to vary

the efficiency (as measured by the ratio of the coincidence rate to the gammaray rate) to obtain an experimental function of beta rate versus efficiency.

This function can then be extrapolated to 100-percent efficiency, and

corrections for the extraneous counts no longer have to be made explicitly.

If the pulse height from the particle detector is monotonical ly related

to the energy of the particle detected, as it can be for liquid scintillators

and high-pressure gas proportional counters, pulse-height discrimination can

be used to vary the efficiency.

For a simple decay, without alternate beta branches or other complications, the extrapolation function of beta-particle count rate versus efficiency should be a straight line. In general the extrapolation function is

taken to be a simple polynomial, with the extrapolated value considered more reliable if a straight line gives a satisfactory fit. For complex decays,

explicit corrections may yet have to be made for differences in branch decay

properties in the extrapolation region. Radionuclides which have no coincident radiations can be "efficiency traced" with quantitative amounts of

another radionuclide which has a similar particle spectrum, but with coincident gamma radiations.

Efficiency extrapolation has become the method of choice for radionuclides to which it can be applied. In an international intercompari son of

i^'+Cs activity measurements organized by the Bureau International des Poids et

Mesures (BIPM) in 1978, the total range of values from 22 laboratories was only 0.7 percent [5].

2.6.4 Sum-Peak Counting . This method, applicable to simple two- or three-photon cascades, also does not use coincidence or anticoincidence

circuitry. The radiations are detected in the same spectrometry system, with

the probability of the simultaneous detection of one of each energy equal to

the product of the efficiencies for detecting each radiation singly. The

result is a spectral peak at the sum of their energies. The areas of the

single-photon and sum peaks are used to deduce directly the activity.

2.6.5 High-Efficiency Nal(Tl) Counting . Although individual probabilities per decay and detection efficiencies for gamma rays and x rays in cascade

decays may not be known with sufficient accuracy, the probability of not

detecting any radiation from a decay in large well -type Nal(Tl) detectors may

be small enough that a calibration of suitable uncertainty is possible.

12
2.7 Radioactivity Group Instrumentation at the U.S. National Bureau of

Standards

Some 40 counting systems are in use by the NBS Radioactivity Group for

the direct calibration of radionuclides, checking for impurities, retaining

calibrations, counting special samples, measuring half lives and the probabilities per decay of radiations, and intercompari ng samples of production SRMs.

Some of these systems have been described in the past or are similar to

instruments described by others. In many cases the detector or source-handling

portions were constructed at NBS, but in general commercial modular electronics

are used, with provision for direct or indirect transfer of data to a minicomputer for analysis.

Table 3 lists and describes some of the equipment currently used for

establishing basic calibrations without using cascade radiations. Table 4 is

a similar list for instruments using cascade radiations.

2.8 Present Basic Radionuclide Calibrations at NBS

Finally, Table 5 lists most of the radionuclides for which the NBS Radioactivity Group has developed calibrations in the past three decades, together

with the methods used, the estimated uncertainties as given on recent certificates, and the conformity with results from other national standardizing

laboratories in many cases. NBS uncertainties in Table 5 are given as the sum

of half of the 99-percent confidence interval of the mean, plus the linear sum

of estimates of the limits of suspected systematic uncertainties [4].

Some radionuclides have been compared bilaterally with other nations, but

for gamma-ray-emitting radionuclides the ionization chambers maintained at the

Bureau International des Poids et Mesures have proven an efficient mechanism.

NBS has 27 radionuclides recorded on these chambers at the present time. The

uncertainties shown for the international i ntercompari sons in Table 5 are the

simple estimated standard deviations of the means.

3, Principles of Operation

3.1 Construction of the NBS Ionization Chamber

The detecting element is a "4tt" (re-entrant) cylindrical ionization chamber

in which the current produced by gamma rays from the sample is compared with

current from long-lived reference sources (see fig. 1). The chamber used for

most of the calibration services was constructed at the Chalk River Nuclear

Laboratories of Atomic Energy of Canada, Limited, but a commercial version from

20th Century Electronics, Ltd. in England is used in a later automatic-samplechanging system on which basic NBS calibrations are also being stored. The

automatic chambers may later be used in the calibration service in the same way

as the present manual chamber.

All chambers are filled with 20 atmospheres of dry argon. A 4-inch-thick

lead housing surrounds the chamber presently used for service calibrations to

reduce environmental background effects and prevent a change of efficiency from

movable scattering material in the vicinity. The voltage for this chamber is

600 volts. A rigid tight-fitting holder made of low-Z material is used for the

samples and reference sources to insure reproducibility of position.

13
TABLE 3 INSTRUMENTS FOR NBS BASIC CALIBRATIONS NOT USING CASCADE RADIATIONS

Connting System Label Description and References

O.lna scintillation

counter

O.lna A carefully measured external baffle about

11 cm in diameter establishes the solid

angle subtended by a CaF(Eu) scintillator

in an evacuated tube. As with the

following alpha-particle-counting systems,

extrapolation to zero energy is used to

account for the small fraction of the

alpha spectrum masked by noise or the

detection of recoil nuclei or other

radiations

0.87t Robinson a scintillation

counter

O.Sna The solid angle subtended by a CsI(Tl)

scintillator greater than 15cm in diameter

is limited by a precise hole in a diaphragm

and by a central stepped cone. The latter

minimizes corrections for source size (up

to 1.6-cm diameter) and displacement.

Intervening air is replaced by hydrogen to

reduce energy losses.

2na proportional

counter

2na A 12-cm-diameter hemispherical counter

with a wire-loop electrode is used for

measuring the alpha-particle-emission rate

of thin sources up to 9 cm in diameter.

Measured and calculated effects of source thickness permit calculation of activity

from alpha-particle-emission rate.

Liquid scintillation

counters

LSa A commercial sample-changing system

with two refrigerated phototubes

viewing lO-ml vials. Outputs of the

two tubes are summed, with a coincidence

requirement. Pulses are sent through

an external amplifier and then analyzed.

LSb

LSc

A commercial sample-changing system

with refrigerated phototubes,

logarithmic amplifier, and microprocessor

control and analysis.

A refrigerated NBS-built system with

the sample in a quartz spectrophotometer

cell optically coupled to a phototube

on each end.

LSd The "LS Cascade" system but without

gamma-ray-detection requirements.

14
4n proportional

counter

47rp

Table 3 cent.

A metallized film carrying a thin source forms the boundry between two hemispherical

proportional counters, each with a loop

electrode. It is used primarily for

high-energy pure-beta-ray-emitt ing

radionuclides.

Pressurized 4n

proportional

counter

PPCX Similar to the above 4n proportional

counter, but modified for pressures of a few atmospheres. It is used for measuring

x-ray-emission rates of pure electron

capture radionuclides of low atomic number.

Radon pulseionization

chambers

Radon Radon gas is quantitatively flushed from

PIC radium solutions or introduced from a collection flask. Water and oxygen are removed, and the gas transferred to one four 3.6-1 cylindrical pulse-ionization

counters.

of

Microcalorimeter Cal A twin-cup radiation balance is used to

compare compact similar sources. Radionuclides with radiations that can be

totally absorbed are compared with a Peltier current.

Internal-gas

counters

Gas Radioactive gases are quantitatively

mixed with counting gas and introduced

into a common chamber containing 3 copper-walled proportional counters of

different length and a comparable set

of 3 counters with walls of stainless steel.

Spectral extrapolation of the difference

in counting rates for coxinters of different

length is used.

Def ined-geometry

system for

low-energy photons

Photon A 0.015-cm-thick Pt diaphragm with a Def. hole 0.9505 ± 0.0020 cm in diameter

Geom. quantitatively establishes the area subtended by a 1.6-mm-thick Nal(Tl)

crystal with a 0.13-mm-thick Be window.

Carefully measured source positions are arranged in a vacuum housing containing

the apparatus. The emission rate of

photons between 5 and 40 keV can be

measured with an uncertainty of less than

one percent.

Pin-well Nal(Tl) Pin Well Two-inch-diameter Nal(Tl) detectors with

thin-walled wells or central passages less

than 1 cm in diameter are used for measuring

photon-emission rates in the energy region

of 50 to 150 keV.

15
TABLE 4 INSTRUMENTS FOR BASIC NBS CALIBRATIONS USING CASCADE RADIATIONS

Cotinting System Label Description and References

Thin-crystal

scint illation

system

Large sodium

iodide system

Liquid scintillation

system

Thin-Xtal A Nal(Tl) crystal 1.6mm thick is used

Cascade with a thin plastic scintillator for

measurement of gamma rays in coincidence

with low-energy beta rays, or in summation

with another thin Nal(Tl) crystal for

sum-peak counting of ^-^^I.

8" Nal Two 8" by 4" Nal(Tl) crystals with

shallow wells are brought together to form

a system for sum-peak or high-efficiency

cascade counting.

LS A hemisphere containing a liquidCascade scintillation cocktail is optically coupled

to the phototube on which it sits. A 4"

Nal(Tl) crystal with a 2" well is

inverted over the hemisphere. Coincidence

or anticoincidences are used to measure

the efficiency for selected beta or electron-capture branches as a function of

energy.

Early atmospheric

proportional counter - gamma-ray coincidence

system

Atmospheric

proportional

counter system

with source changer

Pre ssure-proportional

counter system

PC-y A spherical counter has opposing wire

loops in top and bottom, with the source

at the center plane on a gold-coated

collodion film at ground potential. The

2-in X 2-in Nal(Tl) crystal is external

to the counter.

PC W-heel Two "pill-box" proportional counters

operating at atmospheric pressure are separated by a thin metallized film carried

in a slide. Shielded 3" x 3" Nal(Tl)

gamma-ray detectors are above and below the

proportional counters. Coincidences are recorded. An automatic sample changer

inserts in turn one of 30 samples. Cover

films are added for efficiency extrapolation.

PPC Two "pill-box" proportional counters

Cascade operating at pressures of about 1.5 MPa are separated by a slide carrying a metalized

thin film on which the source is deposited.

Shielded 3" by 3" gamma-ray detectors are above and below the proportional counters.

The system is operated in the coincidence

and anticoincidence modes, with discriminator

efficiency variation for extrapolation.

16
o

PiH

oiu

0

O

TiM

W

so

U

«>>»

at

f-H

O

COo

I

o

o

+

O ,H o

+ o

I + o

+

O tHo

+ o

I

ino

o

1

00o

o

+

ino

1

00 M O

M

U

U

u

e

SI Ml

<a a

ft u

CO

iH O rH

COO o

o

T so

rH rH o o

00o rH

00o

in m in

• « • rH o rH o

oM

3

CO

rH V4 u o

B rH

«> H

W

>» ^<*Ho

Q « o •o •o •o es ot M 03 o u

o u O V) M M M M CO cd a

cd OS OS CS U u

U Z U

U u B CO £1, Oi 00

a

o

o

Q

do

o

.aa.

•o

cd u rH u M a> M

Cd Cd U >-

1 U U

u Ph

03

•o

cdu

M

od U

U I

ao

oO

• o

"O

cd » u

M

a Cd o

M o

cd Oi O a*

IO -n

•tH rH

Cd ;3 Z

CdCdrHUkiaU 'Z<V3UStt. SOfeU dooo (S vo rH Tf m VOt-ONO rs cN <N m in m in in in vo

•lH cd o m z U VI Vi >t m m m 00 vo vo 00 00 00

17
tw

(A

5J o

*^ o

C5 OB

(J

M

H o Q o eca

O o u (A 1 OB

G8

B

B ^ c o ?^ 1 1>, >fc 1 L

c: OS § CB o W OS i 1 00

eS <d S S r B 6 nl

c2 cs « o > B o

•o B|o o+ oo+ o m o

cs + > « a o o o Bo

4) 1^ U oo « <M V V

JS

a

cs a,

on

11 41 o o o o * •o o OS a. o B •a u •o O 9 es u cs B u cs I-l o V ^ <n U o M

41 CS hH cs

M

V CS « H cs U u es Z Z » u 1 d o O a. B •»< O a. 00 CU 00 cu O. z

00 o m

«s o « o

•o

cS o o

at ad

asu o U la «> •a cs •o

cs t-<

U cs o cs u

U9 '<-• C/3 u> a .J cs ia % U M <A Or CS a. H o

cs 3

Oi s z

ao

a o < -a < c a 00 .a o H 6 u E l-t CO B M B 00 »\ O r-t ro •-( r-l o o r-l fS (S «s CO

HB

18
ao> «l o El w cl C3 ui 2

1^1

O 9\ 1-1 ON cs <s CO 00 O O o o o o o + -^ + + 1 1 + o u

c;o i

J. o T-1 CS o o o o o ^ o O -"I O rl

00 o

t-( o

ao p~ o "1

f-H i-( tS 1-1

-I 11 o o e ^

•J .-a «» Ho O

•a •a

«a

a

u

u

M «9

ea

u

U

u& o o o o o O o

-3 o T3 •a o •u o •o a og CS a "O « a u u u o ed u u

(A

u

M «9 Wl lA

U

a <A at ca -t

a CS «

CJ CSU

U

a « u >- 2 >- >-

u CJ o O 1 oa 1 1 cu o. Oi Cu s cu V5 cu u u a Cu CU cu 00 Ou Cm C4 Cm cu

aow

03 H o I -a O

OS c

ca o (« o <j u u ca r~ o\ o rr» fn

a

cu 3

•<

0\

ca 0<

CO

19
MOOau

4>B o o+

<4-l o *5

00o o

4>

O

61 + !>

i-t ••-> atO -1-1 .rH CD ^ »dP

,-1

n

4>

O s ^

«>

H

>» o

a> o -HO

at 0

0(i

a

00

O

•o •o s

ct gS

u

uu 1/1

M

M in Oh

ca ed

M

CJ

U Ma

(3O

U 1 U ax •o rH

CLiuK

B si OS

CL, XT 00 CtS

U

J3 1-t 0 H » H CQ Oi

r-( ro r- cs \o O o O o o cs cs

cs CS cs CN cs cs 0

csuSoooaV

t3

•1-t Cmoo aou • u M O

(U P4

I-t (3 a -H

OS •> u S

»-> <^-^ at O

4)d

MO

V) -H

>>

U (A CS u

0) -o >

P4 4> a> I -!-> "O

Qt

U

4)

M C9

,£1 0

-<-» <fl

H

ed

(4-1 <«-4 -M

»-(

O «>

,£)

M 'O

>» -M -U

»-< « ed a a 0 -M -M M M M

20
3.2 Characteristics and Functions

Several important characteristics of the NBS "47r"Y ionization chamber

include: sensitivity, long-term stability, simplicity of operation, and

simple source preparation [2], The instrument is not only used to maintain

results of direct standardization, but also provides checks of consistency

between results of direct measurements made at different times. It is also

useful for testing dilutions and sampling methods. It also serves to check or establish half-life data for most gamma-ray-emitting radionuclides for which

basic calibrations have been performed. All samples measured are carefully

checked for impurities with germanium gamma-ray spectrometers and appropriate

corrections are applied.

3.3 Current Measurements

The electrometer is used as a coul ombmeter, where the voltage developed

across a capacitor is measured [5]. As shown in figure 3 this output voltage

is fed into the comparator, which triggers a timer scalar to start counting

the elapsed time between a low and high voltage reading on the capacitor. The

triggering of the timer scalar is typically one volt to start the scalar and

10 volts to stop it. The critical point is that the comparator circuit is

designed to always measure a fixed difference in voltage, i.e., nine volts,

whether from 1 to 10 or from 2 to 11 volts, in order to minimize temperature

fluctuations in the comparator circuits. After measuring this elapsed time,

the comparator triggers a print control circuit which transmits the timer

scalar and calendar scalar data to the computer for storage and further

processi ng.

"47r" y

IONIZATION

CHAMBER n

H.V.

Multirange

1 Electrometer COMPARATOR

START STOP

SCALER

CALENDAR

SCALER

PRINT- OUT

CONTROLLER

COMPUTER

TERMINAL

COMPUTER

Fig. 3 Schematic Diagram of Electronic System for the "47r"Y Ionization Chamber.

21
The electrometer has a sensitivity stated to be lOyV. The input

amplifier provides high input resistance (2 x 10^'* ohms), low offset current

(less than 5 x lO"!^ A), and low noise. The double-shielded input section

permits floating operations up to 1000 V above chassis. The construction of

the electrometer keeps unshielded capacitance from input HI to chassis ground

below 0.1 pF.

3.4 Source Container and Contents

The solution to be assayed in the NBS "4Tr"Y ionization chamber should

have as nearly as possible the same composition as the standards used to

calibrate the chamber [4]. The gamma-ray-emitting radionuclide solution

should be 5.0 ± 0.2 mL in a standard 5-mL ampoule. The specifications of

these ampoules are as follows:

Where variation in sample volume cannot be avoided, corrections should be

made from data obtained by investigation of changes in the pertinent

parameters [4]. Corrections can be empirically determined for different

solution heights, the function being only slightly dependent on the energy of

the radiation,

4. Operational Procedures

The operational procedures for calibrations using the "4Tr"Y ionization

chamber has been divided into several sections. Each section is designed to

provide instructions for a specific operation. These sections are:

4.1 Description of the actual measurement process

4.2 Receiving instructions for radioactive solution samples

4.3 Step-by-step operating instructions for the NBS "4 ir" Y Pressure

Ionization Chamber

4.4 Data Analysis

4.5 Format of Calibration Report

4.1 Description of the Actual Measurement Process

The stability of this chamber, although excellent, is made moot by

measuring its response to a reference source of 226^3 -jn equilibrium with its

daughters whenever calibrations or measurements are made. The activity of the

sample is taken to be proportional to the ratio of the chamber response

(current) for the sample to that for the reference source. The relative calibration factor, K|^, is given by Rn/R, the ratio of the measured responses of

the 226Ra reference source and a known one bequerel of a stated radionuclide.

body diameter

wall thickness

barium content

16.5 ± 0.5 mm 0.60 ± 0.4 mm

less than 2.5 percent

less than 0.02 percent

trace quantities

lead oxide content

other heavy elements

22
The activity, A, of the radionuclide in a sample, S, to be measured is

given by KDR3, where R5 is the response (current) for the sample relative to

that for the radium source. Background currents measured just before and

after counting the sample and radium must be subtracted from the response for

each. Kf^ diminishes from the value at the time of the original calibration

with the 1600-year half life of the 226^3^ and a correction for this must be

made.

4.2 Receiving Instructions for Radioactive Solution Samples

All radioactive sources arriving at NBS are delivered to the Health

Physics Group where they are examined to determine the radiation level and

possible contamination of containers and packaging.

After Health Physics releases the source to the analyst, the analyst

in turn should follow a quality control procedure to verify that the correct

radionuclide has been shipped in the activity range which has been previously

discussed with the customer. Quality control procedures that should be

implemented by the analyst are as follows:

a. The analyst should re-examine the source for pin holes of any type

and check the ampoule seal.

b. The analyst should inspect the label attached to the primary

container. This helps to insure that it is the required radionuclide

and the activity amount is correct.

c. Technical data sheets and package inserts accompanying the radionuclide package should be read carefully for special instructions

on stability, quality inspection, and chemistry of the product. This

information should conform to the calibration guidelines specifying

the physical and chemical properties of the submitted sample

solution, or see Tables 1 and 2.

Providing that no defects are observed, the sample may be transported for

measurement in the NBS "4Tr"Y ionization chamber.

4.3 Step-by-Step Operating Instructions for the NBS "4Tr"Y Pressure Ionization

Chamber

4.3.1 Starting of the Ionization Chamber System . The system is started

from the control terminal using the following procedure:

Log on as user- "ondata" (lower case)

Password is (blank) , carriage return, "datax" will be displayed

on screen in lower case letters

Type "iconline", carriage return

Answer Y or y for default para., N or n to change and supply answers

(generally use default values)

Carriage return after each answer

23
Questions are as follows:

New Bkg-0; Skip 1

# BKGS=-1

Time: Day#

Input Start of Calender Scalar HR,MIN,SEC

Reset clock on clock scalar

Computer Time: HR:MIN:SEC

Statement of agreement between computer and calender clock

Answer Y or y if OK, N or n to reenter cal . start time

Enter Sample #: 0-BKG, 999-DONE, -999 change CAPR, PTS.

4.3.2 Background Measurements . Background measurements are made

initially and should be examined before continuing. If the background is

inconsistent, or abnormally different, constant repetitive measurements may be

necessary.

4.3.3 Setting the Electrometer . Set the coulomb dial on the

electrometer to 10-^" for background or 10~^ for a sample.

Set the sensitivity dial to the proper setting. The range of sensitivity

is 0.1 to 1000. The sensitivity setting for background is 0.1. Refer to

Table 6 for determining the 1-1000 range settings.

RANGE

i

24
Table 6. Sensitivity Range Settings

Capacitor Ratio Radium Reference Source*

1.0 Background

10.0 3, 10, 20

100.0 20, 60, 100, 200

1000.0 200, 500, 1000, 5000

*The number of the radium reference source used in approximately equivalent to

the number of micrograms of radium per source.

SENSITIVITY

10

1.0 • 100 • •

• 1000

Note: The capacitor ratio and the reference source to be used must be

determined based on the activity provided by the supplier when the sources are submitted, with appropriate corrections for decay.

4.3.4 Measuring Reference Source . The 226R3 reference source is then

placed into a ngid, tight-fitting Lucite holder and lowered into the

chamber.

After the radium reference source is placed in the chamber, change the

timer controller switch from the manual to the automatic mode. This step is

repeated each time a radium reference source or solution sample is measured.

After the designated number of measurements have been made on the 226Ra

reference source, or a sample, type on the terminal "-1", then press line feed

key. This should be done at the end of every set of measurements. This will

display the average count time for the sample after measurement. The sample

number will be requested for the next sample which is to be counted.

Remove the holder from the chamber and replace the 226r/\ reference source

with the sample which is to be assayed.

25
4.3.5 Measuring the Sample . When placing the solution sample in the

chamber, observe the ampoule to make certain that no liquid is in the neck of

the ampoule. The sample is then counted the same number of times in each set

of measurements as the radium reference source. At least two sets of

measurements should be made on the sample, with an equivalent number of

measurements made on the radium reference source.

After all measurements have been completed on the submitted sample and

the radium reference source, it is recommended that background measurements be

repeated.

4.4 Data Analysis

4.4.1 Data Collection . The data collection is under control of a FORTRAN program running on a multi-user super-microcomputer. The start of the

calendar scalar and other default parameters are entered as prompted, by the

operator. The operator enters the sample number, number of data points to be

collected, and capacitor ratio. The data is collected and stored to a disk

for off-line analysis. The programs are available from the Radioactivity

Group, upon request.

4.4.2 Processing the Data . The data processing is done using a FORTRAN

program. The program averages the data for each sample and corrects them for

background and radioactive decay (using the established half life). The

sample activity is calculated using the entered K-value for that radium

reference source producing a current approximating that of the sample. The

results, with intermediate averages if desired, are printed out.

4.4.3 Correction for Impurities . Figure 2a and 2b show the relative

response curves for the NBS "47r"Y pressure ionization chamber for monoenergtic

gamma rays. To construct the response for an impurity radionuclide for which

there is no measured calibration factor, one uses the relative response for

energy and a specific 226^^ reference source to calculate:

where A is the impurity activity, is the reading for the reference source,

R is the reading for the impurity radionuclide emitting gamma rays of energies

with a probability of emission per decay of {Py)r , and (XY)r is the

response for a gamma ray of energy E-j.

The sharp drop-off of response at approximately 100 keV is due to the

absorption of low-energy gamma rays in the 1/8" brass liner in the reentrant

tube in which the source is placed (See fig. 1). The advantage of this liner

is the much lower response to low-energy x-rays, resulting in a more linear

response curve for most nuclides having considerable x-ray emissions.

However, a disadvantage is the low response to nuclides such as ^^^l.

26
4.5 Format of Calibration Report

The following two pages show an example of a Report of Calibration. As

stated in section 1.2, at the end of a calibration exercise, the technical

user will be given a Report of Calibration stating primarily [3]:

a) the principal radionuclide

b) reference time and date

c) method of calibration

d) certified values for activity

e) decay-scheme assumptions

f) assessment of radionucl idic purity

g) overall uncertainty determinations for calibration

5. Assessment of Uncertainty

5.1 Precautions and Uncertainties in Service Calibrations

Measurements performed with the "4it"y ionization chamber ( IC) must be

done under completely fixed conditions. This requires a stable background, no saturation losses due to incomplete charge collection because of recombination

at high rates, and corrections for all impurities. Ampoules must be of the

standard 5-mL type, the volume in ampoules must be 5.0 ± 0.2 mL, and the 226Ra

reference source must be corrected for decay.

Insufficient volume in the NBS standard 5-mL ampoule can be adjusted by

adding an inactive carrier solution to adjust the height of the solution.

Observed variations with height are used to estimate the uncertainty due to

inexact fillings. Uncertainty in activity measurements may be increased if

the contribution of the background is variable, large, or both. The uncer- tainty quoted is derived from measurement of the variations. The operator of

the chamber should make frequent measurements of the background and look for

the source of any unusual variations.

Radionucl idic impurities may or may not have a significant effect,

depending on the radiations of the principal and impurity radionuclide and the

amount of impurity present. If their half-lives are relatively short,

measurements at different times may be used to check consistency. Germanium

detector systems are used to identify and quantify the significant radionuclidic impurities that may be present. The uncertainty in these

measurements is propagated through the calculated effect on the IC results.

The uncertainty in the ratio of radium reference sources used when the

initial sample and test sample differ greatly in activity were determined by

measuring different radium sources against the same and various radionuclides.

27
NBS-443

(Rev. 2-8S)

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

OAITHERSBURQ. MARYLAND 20899

REPORT OF CALIBRATION

for

COMPANY NAME

ADDRESS

R&dionuclide s

Source identification

Source description

Solution composition

ActivityReference time

Overall uncertainty

Photon-emitting impurities

(Activity ratios at

reference time)

Half life

Measuring instrument

Indium-Ill

X5001

Liquid in NBS borosilicate

glass ampoule (D*

Stated to be carrier-free

in 0.1 M HCl

4.157 X 10^ Bq

1200 EST February 19, 1985

0.73 percent

114miii/lllin: (2.0±0.6) x lO"** <3)

2.8048 ± 0.0005 days <^>

NBS pressurized '4n'y ionization

chamber calibrated by 4n(A,x)-Y

coincidence ef f ic iency-extrapolation

Dale D. Hoppes, Group Leader

Radioactivity Group

Center for Radiation Research

Gaithersburg, MD 20899

February 26, 1985

NBS Test No.

Randall S. Caswell, Chief

Nuclear Radiation Division

Center for Radiation Research

*Notes on next page

28
NOTES

Approximately five milliliters of solution. Ampoule specifications

body diameter

vail thickness

barium content

lead oxide content

other heavy elements

16.5 ± 0,5 mm

0.60 ± 0,04 mm

less than 2.5 percent

less than 0.02 percent

trace quantities

The overall uncertainty was formed by taking three times the

quadratic combination of the standard deviations of the mean, or approximations, thereto, for the following:

a) 59 ionization-chamber measurements

on this sample 0,02 percent

b) standard deviation of the mean of

six coincidence measurements 0.06 percent

c) dead time 0.01 percent

d) resolving time 0.02 percent

e) background 0.17 percent

f) gravimetric measurements 0.10 percent

g) standard deviation of the mean of

the original ionization chamber

measurement 0.05 percent

h) photon-emitting impurities 0.03 percent

i) efficiency extrapolation 0.10 percent

j) half-life 0.05 percent

Limits of detection as a percentage of the gamma-ray-emission rate of

the 245-keV gamma rays emitted in the decay of indium-Ill are:

0.1 percent between 30 and 240 keV

0.01 percent between 250 and 1900 keV,

provided that the impurity photons are separated in energy by five keV

or more from photons emitted in the decay of indium-Ill.

NBS-measured half-life value. NCRP Report No. 58, 2nd Edition,

February 1985, lists a half-life value of 2.805 ± 0.001 days.

further technical information please contact J.M. Calhoun at 301-921-23 83.

29
The origin of the uncertainty estimates (as standard deviations of means,

or suitable approximations) in a direct comparison are shown in the following

excerpt from the reporting form for an international intercompari son of

activity measurements of i^sga. Measurement was by anti coi ndences between

electron capture radiations (Auger electrons and x rays) and gamma rays

measured by Nal(Tl) detectors. The efficiency of the proportional counter was varied and an extrapolation of its rate to an efficiency of 100% was made to

give the activity.

i33Ba Uncertainty Components

Due to: Uncertainty

(%)

counting statistics 0.07

weighing 0.1

dead time 0.02

background 0.1

timing 0.004

fitting procedure 0.13

absorption 0.05

radionucl idic impurities 0.001

half-life 0.01

Combined Uncertainty 0.21

Obtained by:

standard deviation of the mean

calculated effect of limits

measured accuracy of live-time

determi nati on

calculation using estimated

background uncertainty

measured oscillator accuracy

error on intercepts

limit of measured effects

limit of i^i+cs fraction

calculated effect of half-life

uncertai nty

(Square root of summed squares)

5.2 Significance of Uncertainty Statements

The uncertainties given in most certificates prepared in past years were divided into "random" and "systematic" parts. The former, derived from the

well-defined estimated standard deviation of the mean of a stated number of

measurements and the assumption of a normal distribution, was given as one- half the 99-percent confidence interval of the mean. The "systematic" part

was given as the linear addition of "limits" of other possible errors in the

measurement system. The "systematic" part was clearly dominant in most radioactivity calibrations. If an overall uncertainty was required, the two parts were added linearly; if the source uncertainty needed to be propagated as a component of the uncertainty in a subsequent measurement, it was sometimes

divided by three and treated as a standard deviation of the mean.

30
Discussions among representatives of several national standardizing

laboratories, following the distribution of a questionnaire about uncertainty

statements by the Bureau International des Poids et Mesures, led to a suggestion that an approach akin to that used for those components susceptible

to ordinary statistical analysis be applied to those that are not. This

suggestion followed at least the spirit of the article by J. W. Mcrller

Ref. [8]. With the idea that even the subsidiary quantities have distributions (perhaps measured poorly or not at all), an approximation of a standard

deviation should be devised for them. A possible virtue is that deducing,

say, the two-thirds coverage of an ill -defined distribution may be less

subjective than setting a "99% limit." Moreover, if all uncertainties are taken to be equivalent to the corresponding standard deviations (or standard

deviations of the mean, if appropriate), the propagation of uncertainties is

straightforward and an approximation to the "total standard deviation of the

mean," called the combined uncertainty, is easily formed. A more conservative

overall uncertainty formed by taking three times this quantity is roughly

equivalent to the former total uncertainty in many cases, and this is done in

present certificates. No matter which method is used, the uncertainty for a radioactivity calibration cannot be considered an accurate quantity, but

rather a best estimate.

5.3 Examples of Uncertainty Components

The uncertainties which have been determined for the NBS ionization

chamber originate predominantly from the direct calibration method for the

particular radionuclide. Uncertainty is also introduced in the operation of

the ionization chamber and in the comparison with radium reference sources to

preserve the initial activity calibration of a specific radionuclide. As

examples, consider two samples submitted to NBS for measurements on the IC.

The overall uncertainty was formed by taking three times the quadratic

combination of the standard deviations of the mean, or approximations thereto,

for the fol 1 owi ng

iiiln

percent

a) 59 ionization-chamber measurements on this sample 0.02

b) standard deviation of the mean of six coincidence measurements 0.06

c) dead time 0.01

d) resolving time 0.02

e) background 0.17

f) gravimetric measurements 0.10

g) standard deviation of the mean of the original ionization

chamber measurements 0.05

photon-emitting impurities 0.03

i) efficiency extrapolation 0.10

j) half-life 0.05

overall uncertainty = 0.73

(3 X /E u2)

31
1231

percent

a) 60 ioni zati on-chamber measurements on this sample 0.01

^) solution composition and density 0.05

c) five coincidence measurements 0.11

d) efficiency extrapolation 0.10

e) gravimetric measurements 0.03

9) resolving time 0.05

h) background 0.01

i) half life 0.20

j) original ionization-chamber measurements 0.01

k) photon-emitting impurities in original calibration 0.04

1) radium 5000 to radium 1000 reference sources ratio 0.25

overall uncertainty = 1.10

(3 X /z u2)

32
6. References

1. NBS Special Publication 260, National Bureau of Standards, Standard

Reference Materials Catalog 1986-1987 ed. [Available from the

Superintendent of Documents, U.S. Government Printing Office, Washington,

D.C. 20402].

2. NBS Special Publication 250, National Bureau of Standards Calibration

Services Users Guide 1986-1988 ed. [Available from the Superintendent of

Documents, U.S. Government Printing Office, Washington, D.C. 20402].

3. Hoppes, D. D., Basic Radionuclide Measurements at the United States

National Bureau of Standards, Environmental Int. J^, 99-107 (1984).

4. NCRP Report 58, A Handbook of Radioactivity Measurements Procedures , National Council on Radiation Protection and Measurement, Bethesda,

Maryland, (1985).

5. Rytz, A., Activity Measurements of a Solution of ^^^cs. Report of an International Conparison, Nucl . Instrum. Methods 192, 427-431 (1982).

6. Keithly Digital Electrometer Instruction Manual, Model 6616, Keithly

Instruments Cleveland, Ohio, April 1976. (Mention of a commercial

product does not imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the product identified is

necessarily the best available for the purpose.)

7. Radioactivity Calibrations Catalog, 1986 (unpublished).

8. Muller, J. W. , J. Nucl. Instrum. Methods 163, 241-253 (1979).

33
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

\HFPl /^o#a in ^tmrt'irin ^) NT^c; /qp_9 SO / in October 1987

4. TITLE AND SUBTITLE

NBS Measurement Services: Radioactivity Calibrations with the "ATT"

Gamma Ionization Chamber and Other Radioactivity Calibration

Capabilities

5. AUTHOR(S)

Jacqueline M. Calhoun

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struct/on sj

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Gaithersburg, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP)

Same as item #6.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 87-619870

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bibliography or literature survey, mention it here)

This paper describes the use of the NBS "4 7r"Y ionization chamber an instrument which provides an indirect method of comparing the activity (decays

per second) of gamma-ray-emitting radionuclides with national standards for a routine calibration service by the National Bureau of Standards Radioactivity

Group. A description of the chamber's construction and characteristics, the

operational procedure, and the associated equipment is included.

A description of NBS capabilities for direct radioactivity calibrations

is also presented. Many of these capabilities are used to establish

calibration factors for the "4 it"y ionization chamber.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

activity; calibration; gamma ray; ionization chamber; radionuclides; radium

reference source; standard

13. AVAILABILITY

[X] Unl imited

I I For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 2216 I

14. NO. OF

PRINTED PAGES

41

15. Price

USCOMM-DC 6043-P80


PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

SP 250-1 NBS Measurement Services: SP 250-12

Spectral Radiance Calibrations

J.H. Walker, R.D. Saunders, and A.T. Hattenburg

SP 250-2 NBS Measurement Services: SP 250-13

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

SP 250-3 NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet SP 250-14

J.Z. Klose, J.M. Bridges, and W.R. Ott

SP 250-4 NBS Measurement Services:

Irradiation of Ferrous-Ferric Dosimeters

(Fricke- Dosimetry) SP 250-15

C.G. Soares, E.L. Bright and M. Ehrlich

SP 250-5 NBS Measurement Services:

Alpha-Particle Calibrations SP 250-16

J.M.R. Hutchinson

SP 250-6 NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J. Hsia, K.D. Mielenz, SP 250-17

and V.R. Weidner

SP 250-7 NBS Measurement Services:

Radiance Temperature Calibrations

W.R. Waters, J.H. Walker, and A.T. Hattenburg SP 250-18

SP 250-8 NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J. Hsia SP 250-19

SP 250-9 NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt SP 250-20

NBS Measurement Services:

Radioactivity Calibrations with the "4Tr" Gamma Ionization Chamber and Other

Radioactivity Calibration Capabilities SP 250-21

J.M. Calhoun

NBS Measurement Services:

Dosimetry for High-Dose Applications

J.C. Humphreys, D. Hocken, and W.L. McLaughlin

NBS Measurement Services: Neutron Personnel Dosimetry

R.B. Schwartz

NBS Measurement Services:

Activation Foil Irradiation

with Cal iforniiin Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Activation Foil Irradiation by Reactor Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Photometric Calibrations

R.L. Booker and D.A. McSparron

NBS Measurement Services:

Calibration of X-Ray and Gamma-Ray

Measuring Instruments

P.J. Lamperti, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

NBS Measurement Services: Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

J.T. Weaver, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

Spectral Irradiance Calibrations

J.H. Walker, R.D. Saunders, J.K. Jackson,

and D.A. McSparron

NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrumentation and Sources

J.S. Pruitt, C.G. Soares, and M. Ehrlich
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300